As a preface and guide, the following acronyms and terms used in this disclosure represent the associated definitive terminology below:
______________________________________ AMPS Advanced Mobile Phone System BER Bit Error Rate BSC Base Station Controller BSS Base Station Subsystem (a combination of a BSC and one or more associated BTSs) BTS Base station Transceiver Subsystem BTS-A Base station Transceiver Subsystem of cellular communication signal format A (PCS1900) BTS-B Base station Transceiver Subsystem of cellular communication signal format B (IS-661) CDMA Code Division Multiple Access (a communication channel scheme where a each channel has its own code and a MS only accepts information including a preassigned code) dual mode MS (mobile station) having circuitry such that it is able to MS operate in two different mode signal formats such as PCS1900 and IS-661 DS1 Signal transmission link having a information capacity of 24 message channels and has a bandwidth of 1.544 Mb/s FDMA Frequency Division Multiple Access (a communication channel scheme where a each channel has its own frequency and a MS only accepts information on a preassigned frequency) foreign A different system. The difference may be in format, system territory, ownership etc. GSM Global System for Mobile Communications handoff Act of transferring communication with a mobile station from one base station to another base station where both base stations use the same communication signal format even though they may be connected to different MSCs or even a foreign MSC involving an interexchange or intersystem handoff handover Act of transferring communication with a mobile station from one base station to another (foreign base station) where the two base stations use different communication signal formats even though they may be connected to the same master switch HLR Home Location Register ID IDentification IS-661 PCS (personalized cellular communication system) standard that uses a combination of TDMA, FDMA and CDMA for differentiating and optimizing the communication channels mobile Cellular system where handoff of an active call of that initiated system is initiated by a MS network Cellular system where handoff of an active call of that initiated system is initiated by a portion of the network other than the mobile station MS Mobile Station (includes cordless and cellular telephones, radios, fax devices etc.) MSC Mobile services Switching Center OAM Operations, Administration and Management PCS Personal Communication Service (a personalized cellular communication system) PCS-1900 Digital cellular system for the US market, similar in format to a GSM system, using TDMA and FDMA in the communication channel process and operating in the 1850-1990 MHz bands PSTN Public Switching Telephone Network TDMA Time Division Multiple Access (a communication channel scheme where a each channel has its own time slot of occurrence with respect to a reference and a MS only accepts information during a preassigned time slot) VLR Visitor Location Register ______________________________________
The most prevalent form of a mobile wireless communication system is a cellular network. In such a network, a territory serviced by it is divided into a plurality of geographically substantially distinct, but normally overlapping cells. Within each cell is a BTS (Base station Transceiver Subsystem) at which there is an antenna or antenna array connected to a bank of radio transmitters and receivers (hereinafter "radios") for communicating with MSs (Mobile Stations) within the territory. A plurality of BTSs in a GSM (Global System for Mobile Communications) type system are controlled, at a first level of control, by a BSC (Base Station Controller). Such a BSC and its associated BTSs may be termed a BSS (Base Station Subsystem). A plurality of these BSSs are connected to a switch sometimes referred to as a MSC (Mobile services Switching Center). This connection is often a communication link having the capacity of at least a DS1. Within the MSC and the BSC there are a plurality of subsystems for routing calls to appropriate BTSs, for issuing instructions to the connected BTSs, and administrative duties like keeping track of home registered MSs as well as of visiting MSs.
The MSC connects calls between two MSs within the network, between a MS and a PSTN (Public Switching Telephone Network) and periodically between a MS of that system and a MS of a foreign mobile system. When two MSs need to communicate with each other and are within cells controlled by a single BSC, the BSC is typically able to make the connection without communicating that action to the MSC.
As is known by those skilled in the art, there are many different signal formats used for communicating with an MS. The format difference may be a variation in the way in which information channel are changed in frequency, in time of occurrence and in the way the data is modulated or otherwise coded into a given information channel. Some example formats are FDMA (Frequency Division Multiple Access), TDMA (Time Division Multiple Access), CDMA (Code Division Multiple Access), and combinations thereof.
The analog cellular systems in the United States started operation in the 824-849 MHz frequency bands. Present day cellular systems in the U.S., many of which are digital, still use the original frequency bands and also operate in the 1850-1990 MHz frequency bands. However each MSC has, regardless of the communication format used with the MS, similar functions in that it has to be able to establish communication with MSs in all the cells for which that MSC is responsible so that appropriate communication channels can be assigned and later reused. The MSC further needs to recognize and track foreign system MSs that enter a cell for which that MSC is responsible.
As mentioned above, the term "handoff" refers to the transferring of communications between an MS and the system from the BTS of one cell to another BTS. An interexchange handoff may occur to the BTS of a "foreign" system cell and, in such a situation, communication must take place between foreign and local MSCs. When it is obvious to which adjacent cell a MS is being transferred, it is referred to as a "two way handoff." However, at times, a MS approaches the influence of two adjacent cells simultaneously, and, thus, the MSC must prepare both of the affected BTSs to receive the incoming MS because the direction that the MS takes upon leaving a given cell is not predictable. This handoff is designated in the prior art as a "three way handoff." Once the MS is communicating with one of the two possible receiving cells, the communication channels reserved in the original and alternate receiving cells are released for reuse by other MSs.
The original MSs in analog systems were busy 100% of the time with a call, when in an "active call" mode, and all decisions as to when a call should be handed off to another cell and to which cell it should be handed off, were made by system controllers, such as a MSC, a BSC or one or more BTSs acting in concert. Some present day digital MSs are busy communicating with the local BTS a fraction of the duration of an active call. As an example, the TDMA used by some GSM type systems utilizes eight time slots. Thus the MS is communicating with a local BTS only during one time slot for receiving voice information and another time slot for transmitting voice information. The MS thus has a considerable amount of time (the other six time slots) to transmit, receive and/or process other information or perform other functions. The MSs, in prior art GSM type systems, have used this time to monitor the reception of signals from other nearby BTSs and measure signal quality in the form of signal strength and in some systems BER (Bit Error Rate). Typically the MS has recorded and stored not only this information, but, in addition, the ID (IDentification) of the BTS being measured. This information is then periodically transferred to the appropriate system controllers so that the system can determine when a handoff should occur. This handoff determination is referred to as network initiated since a centralized controller is the arbiter of the handoff process.
Another digital PCS system known as IS-661 has 16 or 32 time slots. In the 16 time slot version, the MS uses 1 of the 16 time slots for sending user generated signals to the BTS and 1 additional time slot for receiving signals from the BTS. The MS thus is in communication with a BTS handling the active call only a small percentage of the time. Since the system design (combining TDMA, CDMA and FDMA to be used simultaneously) allowed many more communication channels to be handled simultaneously by a single BTS, the system design made the MS intelligent and let it request that a handoff be permitted to occur. The handoff request would be determined by the quality of other signals received by the MS as compared to the quality of the active call signals. As is known to those skilled in the art, fading of a signal in an analog system is typically very gradual as compared to a digital system. When a MS is capable of monitoring the signal quality of many nearby stations and can keep this information in readily updated storage, it may very well be the best resource for determining when a handoff should occur and to which BTS. This type of handoff determination is referred to as mobile initiated.
A dead spot in a cellular system defines an area, within a given cell, wherein there is no reliable communications between an MS and the BTS defining that given cell. After a cellular system is designed and activated, conditions can change to cause dead spots in a cell which were not there previously. This may be caused by a new building, changes in landscape due to road construction etc. While the communication problems caused by such dead spots can often be overcome by handoffs to other BTSs in the general area or by additional low power and strategically located antennas, there are occasions such system modifications are not economically practical.
Another problem that may occur after a cellular system is completed is that the number of MSs brought into a given area and attempting communications at a given time far exceeds expectations. This problem can sometimes be overcome by subdividing the original cell into many additional new cells with the accompanying cost of additional cell sites, additional BTSs and invariably additional switching capacity.
A further problem may be caused by a building or set of buildings, such as an educational campus, which may wish to have an internal communication link with cordless personal communication units similar to a PCS for communication within the campus via a private branch exchange (PBX) system having low power antenna apparatus that is substantially confined to the campus. Such a private system can be very cost-effective for the campus and ease the problems of cellular system expansion by the local cellular provider. When one of these units needs to communicate with phones outside the confines of the campus, the PBX may connect to the outside world via a communication link to a local cellular provider. If the campus system is a completely self contained, it may connect directly to a public service telephone network. Regardless of the inner workings of such a campus system, it would be convenient to be able to use such a cordless personal communication unit outside the campus and not have to buy and/or carry an additional MS. It would be even more convenient if one could commence a conversation via the campus system and continue the call seamlessly to a local cellular system when the MS leaves the confines of the campus. With such a system, the caller would only be charged for phone call time while beyond the confines of the campus.
Even though the cellular phone units in the campus situation described above can be moved, the associated system is known in the art as a "fixed access unit" when the system has a non-movable antenna attached to the building and it serves one or more phone units. In a campus situation, the system would typically include a BTS to serve multiple phone units. Further, such a system would additionally typically support many phone units on campus which are wired in place rather than being cordless or cellular.
As a separate issue, it may be noted that dual mode MSs have been known in the prior art which may allow an active call to be handed off between a TDMA system and an AMPS (Advance Mobile Phone System) system. Both of these systems are network initiated and in the situations where such dual mode MSs were used, the systems were adjacent each other and one did not overlay or super impose upon the other.
Further problems with prior art systems included the time involved in determining that a handoff was required, determining to which BTS the handoff should go, allocating resources and sending the appropriate information to the MS. With different systems and different signal formats, the coordination problems involved in a handoff between systems were substantial.